1. Field of the Invention
The present invention relates to a substrate having an active element array, comprising a quartz glass substrate and active elements formed on the quartz glass substrate, such as a TFT substrate for driving a liquid crystal display of an active matrix type, and, more specifically, to the composition of the quartz glass substrate.
2. Description of Related Art
The substrate having active element array has been used as a principal component of various devices. For example, a substrate having thin-film transistors (TFT substrate) provided with an integrated circuit of thin-film transistors, i.e., active elements, is used as a driving substrate for driving a liquid crystal display of an active matrix type. The display quality of the liquid crystal display is quite sensitive to the quality of the glass substrate of the TFT substrate, and the quality of the glass substrate influences the element fabricating process. Accordingly, improvement of techniques relating to the glass substrate is important for the development of liquid crystal displays. The requisite characteristics and quality of the glass substrate will be briefly described hereinafter.
First, the glass substrate must have a flat surface. The flatness of the surface of the glass substrate is deteriorated by warps, which are upward and downward flexures that appear in the surface of the glass substrate. If the glass substrate is warped, problems arise in fastening the glass substrate on a working table with a vacuum chuck and in patterning thin films by photolithography to form minute TFTs. If a liquid crystal display is provided with a substrate having an active element array using a warped glass substrate, unacceptable irregular distribution of color on the screen occurs.
The glass substrate must have high heat resistance. The maximum withstand temperature of the glass substrate, in general, is represented by the softening point. The softening point must be higher than the maximum process temperature in the liquid crystal display manufacturing processes. For example, in a process of manufacturing a semiconductor device for processing a polycrystalline silicon film to form TFTs, the process temperature is 1000.degree. C. or higher. Therefore, the glass substrate must have high heat resistance. From such a point of view, a quartz glass substrate is used widely. The use of molten quartz glass is described, for example, in Nippon Gakujutsu Shinko-Kai Committee No. 142, "Handbook of Liquid Crystal Devices", Nikkan Kogyo-sha, p. 22, 1989.
The glass substrate must have high purity. If metal impurities contained in the glass substrate, such as alkaline components of the glass substrate, dissolve into liquid crystal cells, the resistivity of the liquid crystal is reduced and the display characteristics of the liquid crystal cell changes. If a small amount of alkaline components of the glass substrate dissolves into a functional film, such as a polycrystalline silicon film, in forming TFTs, the characteristics of the functional film may be deteriorated.
As is obvious from the foregoing explanation, a glass plate to be used as a substrate for supporting active elements must have high quality in respect of flatness, heat resistance and purity. To meet such requirements, quartz glass substrates are used. The characteristics of quartz glass is greatly dependent on the method of manufacturing quartz glass. Quartz glass is classified roughly into two kinds of quartz glass; natural quartz glass obtained by melting natural quartz at high temperature and casting the molten natural quartz, and synthetic quartz glass obtained by depositing source materials by a vapor deposition process, such as a CVD process, using SiCl.sub.4, H.sub.2 and O.sub.2 as materials.
However, the prior art quartz glass is not necessarily satisfactory as a material for forming a substrate for supporting active elements in respect of high purity and high heat resistance. Although the molten quartz glass has a high softening point in the range of 1050.degree. C. to 1100.degree. C. meeting a required heat resistance, the molten quartz glass contains alkaline metal impurities, such as Na, K and Li, unavoidably, because the the molten quartz glass is obtained by melting natural quartz.
Although the synthetic quartz glass has very high purity, the heat resistance thereof is not satisfactory. A substrate of the synthetic quartz glass is subject to warping and deformation under high temperature.
Various measures have been proposed to improve the heat resistance of synthetic quartz glass. For example, a technique disclosed in Japanese Patent Laid-open (Kokai) No. Hei 3-193637 adds aluminum to high-purity synthetic quartz glass. However, the addition of an impurity to quartz originally consisting of pure SiO.sub.2 requires complicated processes and the impurity may possibly cause adverse effect.